Because of their strength and clarity, polycarbonate resins have a great many significant commercial applications. Unfortunately, polycarbonate resins are inherently flammable and can drip hot molten material causing nearby materials to catch fire as well. Thus, in a number of applications employing polycarbonate blends, it may be desirable to include additives which retard the flammability of the material and/or which reduce dripping. The challenge is to identify additives which accomplish this purpose without compromising the desirable properties of strength and clarity, without introducing new problems (such as the potential environmental problems associated with halogenated additives) and without prohibitively increasing the price.
A variety of different materials have been described for use in producing fire-retarded and/or drip-resistant polycarbonates. Exemplary of these are the materials described in U.S. Pat. Nos. 3,971,756, 4,028,297, 4,110,299, 4,130,530, 4,303,575, 4,335,038, 4,552,911, 4,916,194, 5,218,027 and 5,508,323. Notwithstanding these varied disclosures, however, there remains room for improvement in the formulation of fire-retarded polycarbonate resin.
Achieving flame-retardant resin properties sufficient for obtaining an Underwriter's Laboratories listing is a prerequisite for many commercial applications of thermoplastic resins. To achieve listings of UL94 V-1 and or UL94 V-0 at the desired thicknesses (≦2.5 mm), several known flame retardants can be used. But in addition to adding cost, they have other disadvantages compared to non-flame retarded compositions. For example, the use of brominated compounds would preclude a resin from use in applications where conformance with TCO'99 guidelines (non-halogen) is desired. Other regulations or customer preferences may preclude the use of elemental (red) phosphorous or organic phosphates and phosphites. In addition, organic phosphates and phosphites reduce the deflection temperature under load (heat distortion temperature) of polycarbonate (PC)-resins and—because they are low molecular weight compounds—can form undesirable deposits on injection molding tools or the part surface, which in turn can lead to chemical stress cracking. Melamine cyanurates or inorganic hydrates (like aluminum trihydrate—ATH) cannot be incorporated effectively into resins requiring high processing temperatures due to the onset of their own thermal decomposition in that temperature range and typically reduce impact resistance significantly due to their particulate nature. These constraints are even more important in fiber reinforced composites, because these typically require larger amounts of flame retardant or more effective/costly ones to achieve UL94 V-1 or even UL94 V-0 listings.
Among the additives which are widely used commercially in fire-retarded polycarbonate resin compositions are organic salts, particularly sulfonic acid salts. Particular examples of these salts are perfluoroalkane sulfonates, such as potassium perfluorobutane sulfonate (“PFBS”, also known as “Rimar salt” and potassium diphenylsulfone sulfonate (“KSS”), which may yield haze free compositions when blended with polycarbonate resin. The use of perfluoroalkane sulfonates in polycarbonate resins is described in U.S. Pat. No. 3,775,367. However, the benefits which can be obtained using these materials alone are limited and indeed additional additives are generally included.
Besides perfluorinated salts such as “Rimar Salt” (potassium perfluorobutane sulfonate, PFBS), organo-silicones may be used to help solve the above stated problems. However, like the above mentioned flame retardant (FR-) technologies, their efficiency (concentration required for V-0 or V-1 capability at a given thickness) is greatly reduced in fiber reinforced polycarbonate (PC) compositions, leading to increased cost and secondary effects of the FR such as reduced hydrolytic stability in the case of Rimar Salt, decreasing modulus and heat distortion temperature in the case of the organo-silicone. Organo-/nano-clays have also been demonstrated to have a beneficial effect on the FR-properties; however, most nano-clays commercially available today contain highly basic organic moieties which degrade the molecular weight of PC to the point of making the nano-clays practically not usable in PC.
There is still a need for an alternative to the above mentioned flame retardant technologies.